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An international collaboration which was headed by Indian researchers discovered that eRASSU J060839.5–704014, also called eRASSU J0608, is one of the fastest-evolving ultracompact double white dwarf binaries. The two white dwarfs in this system orbit each other once every 374.15013 seconds, or around 6.24 minutes. This result, published in The Astrophysical Journal Letters on August 10, 2026, suggested that the orbit is decaying at an abnormally fast rate, thus making it suitable for gravitational-wave astrophysics.
Two White Dwarfs Orbit Each Other in 374 Seconds
White dwarfs are dense remnants of stars formed through shedding of their outer envelopes after exhausting all nuclear fuel.
In an ultracompact binary, the two white dwarfs can revolve very close to each other. For eRASSU J0608, the period of orbiting is 374.15013 seconds.
This object has been observed first using eROSITA, the X-ray instrument in the Spektrum-Roentgen-Gamma mission as a supersoft X-ray source. Later, it has been shown to be a double degenerate ultracompact binary in the foreground of the Large Magellanic Cloud.
Direct-Impact Accretion Generates X-Rays
It is a rare binary system where matter from one white dwarf strikes its partner directly without forming an accretion disk.
The term used for this phenomenon is direct-impact accretion.
The X-ray data confirm this idea. Scientists detected the presence of two thermal components at the temperatures of 126 and 144 electron volts, or 1.5 million and 1.7 million kelvins, respectively.
Direct-impact accretion results in a very hot area formed due to the effect of incoming matter on the target white dwarf.
The Orbit Is Dwindling at an Alarming Rate
The team observed the binary for around three-and-a-half years using data from,
- NASA’s NICER
- China’s Einstein Probe
- ESA’s XMM-Newton
They observed that the rate of decrease in the orbital period was about 4.7 × 10⁻¹¹ seconds per second.
The orbital decay is greater than the decay rate in two other famous ultracompact double-degenerate binaries, HM Cnc and V407 Vul.
According to the team, the high rate of evolution can be attributed to the loss of orbital energy and angular momentum through gravitational radiation. Considering the gravitational wave emission process is responsible for the same, the chirp mass of the system can be calculated to be approximately 0.43 solar masses.
What is the Chirp Mass?
The chirp mass is a quantity that is derived from the masses of the two stars and plays an important role in the frequency and amplitude of the waves emitted by the system.
The estimate of the chirp mass at 0.43 solar masses makes eRASSU J0608 one of the more massive members of this class of objects, as the authors point out.
This quick orbital evolution allows us to study a very brief phase of evolution in extremely compact stellar binaries.
Possible Verification Source for LISA
It can serve as a verification source for any future detection of gravitational waves.
A verification source is a binary system whose gravitational wave emission can be determined separately through astronomical observations. This prediction can then be compared with the detection of gravitational waves using a gravitational wave observatory.
One of the possible future gravitational wave observatories will be the Laser Interferometer Space Antenna (LISA). This is a space mission of the European Space Agency that will detect low-frequency gravitational waves.
It will consist of three satellites separated by millions of kilometers. The ESA plans to launch the mission in 2035.
Distance Still Remains an Uncertain Factor
Currently, the exact distance to eRASSU J0608 has not been determined yet. What is known is that the star is located in the region of the Large Magellanic Cloud but is considered to be closer than the latter.
Having the better knowledge of the distance to this object will allow getting more accurate information about its physical characteristics and gravitational wave signature.
More X-ray observations of this source and possible identification of a visual counterpart are planned by the research team.
